Urticaria is a common skin disorder involving mast cell activation and degranulation. Urticaria is classified according to its chronicity into acute and chronic forms. It may occur spontaneously or on exposure to a physical factor. In the latter case, the urticaria is classified as a physical urticaria. Physical urticaria may be induced by mechanical and applied pressure, exercise, or exposure to cold, heat, sun, water, or vibration. Passive transfer experiments, whereupon serum from affected donors is transferred into recipients'skin followed by physical stimulation has been positive in some instances with resultant urticaria at the site of challenge. This suggests the presence of an intrinsic factor in serum, such as IgE, which then mediates activation of tissue mast cells. However, the pathogenesis in general remains unclear and a genetic basis for these disorders has not been elucidated. The purpose of this protocol is to investigate the mast cell dependent pathogenic mechanisms of physical urticaria both to better understand how to manage urticarial inflammation and to explore the consequences of mast cell degranulation in human tissues. In these studies adult and pediatric patients undergo standard challenge testing to verify their urticaria. Blood samples are obtained for the investigation of molecular and genetic pathways involved in the disease process. Following the clinical induction of urticarial manifestations, additional blood samples are collected to determine soluble mediators involved in pathogenesis. Photographic imaging studies are performed during challenge testing. Skin biopsies are obtained prior to and following challenge testing that are analyzed for biochemical and histological markers. Patients receive counseling regarding appropriate management of their disease and may be scheduled for a return visit to provide additional samples and for longitudinal analysis of their disease manifestations. Blood and tissue may be stored for future studies. Since the inception of the physical urticaria protocol in June 2009 we have enrolled 74 patients (19 control patients). All patients safely underwent challenge testing based on their history. Blood samples and skin biopsies have been collected and stored for biochemical, molecular and when applicable, genetic analysis. Mast cell degranulation was verified by skin biopsy. The majority of the patients were challenge positive to either cold-induced, cholinergic, dermatographism, solar or vibratory urticaria. To date, in 15 patients with physical urticaria, we have found IL-1 beta levels from stimulated whole blood samples not to be elevated, suggesting lack of activation of the inflammasome in this condition. In seven patients with acquired cold urticaria we have found a consistent peak in serum histamine at approximately 10 minutes following cold hand immersion challenge testing which correlates with the regional changes in dermal blood flow as determined by non-invasive imaging technology. Serum tryptase was not elevated. Other mediator assays are pending. Antihistamine variably and incompletely blocked changes in blood flow, suggesting a role for yet-to-be identified mediators.